Stents, which are generally open tubular structures, have become increasingly important in medical procedures to restore the function of body lumens. Stents are now commonly used in translumenial procedures such as angioplasty to restore an adequate blood flow to the heart. However, stents may stimulate foreign body reactions that result in thrombosis or restenosis. To avoid these complications a variety of stent coatings and compositions have been proposed in the literature both to reduce the incidence of these complications or other complications and restore tissue function by itself or by delivering therapeutic compound to the lumen.
Stents generally are coated by simple dip or spray coating of the stent with polymer or polymer and a pharmaceutical/therapeutic agent or drug. These methods are acceptable for early stent designs that were of open construction fabricated from wires (Wiktor stent) or from ribbons (Gianturco). Dip coating with relatively low coating weights (about 4% polymer) could successfully coat such stents without any problems such as excess coating bridging (i.e. forming a film across) the open space between structural members of the device. This bridging is of particular concern when coating more modern stents that are of less open construction, such as the Palmaz-Schatz, Crown, Multilink or GFX stents. Bridging of the open space (slots) is undesirable because it can interfere with the mechanical performance of the stent, such as expansion during deployment in a vessel lumen. Bridges may rupture upon expansion and provide sites that activate platelet deposition by creating flow disturbances in the adjacent hemodynamic environment or pieces of the bridging film may break off and cause further complications. Bridging of the open slots may also prevent endothelial cell migration complicating the endothelial cell encapsulation of the stent.
Similarly, spray coating can be problematic in that there is a significant amount of spray lost during the process and many of the pharmaceutical agents that one would like to incorporate in the device are quite costly. In addition, in some cases it would be desirable to provide coated stents with high levels of coating and drug. High concentration coatings (.about.15% polymer with additional drug) are the preferred means to achieve high drug loading. Multiple dip-coating has been described in the literature as a means to build thicker coatings on the stent. However, composition and phase dispersion of the pharmaceutical agents affect sustained release. In addition, the application of multiple dip coats from low concentration solutions often has the effect of reaching a limiting loading level as an equilibrium is reached between the solution concentration and the amount of coating, with or without pharmaceutical agent, deposited on the stent.